The main reason is that is the DoD standard.
The secondary reason is, the potential for several eye issues increase above -5.00. So, the standards is generous indeed
Just to provide additional insight into why the standard of +/- 8.00 is there along with the fact that it doesn't matter if you subsequently get LASIK/PRK is that those "issues" alluded to by Mr. Mullen in his response are not treated by or the risk reduced by having refractive surgery. In fact for many of those issues PRK/LASIK/etc can INCREASE the risk of problems.
There is also a very much increased risk of complications from having LASIK/PRK for those who are considered "high myopes" (those beyond that 8 range).
Anyway, hope that helps provide a little more knowledge as most people would think "my eyes are beyond 8, I get them 'fixed' and now I'm 20/20, what's the big deal?"